Oncogenes are genes that can cause cancer. Carcinogenesis can occur by a wide variety of mechanisms, including infection of cells by viruses containing oncogenes, activation of protooncogenes (normal genes that have the potential to become an oncogene) in the host genome, and mutations of protooncogenes and tumour suppressor genes. Carcinogenesis is fundamentally driven by somatic cell evolution (i.e. mutation and natural selection of variants with progressive loss of growth control). The genes that serve as targets for these somatic mutations are classified as either protooncogenes or tumour suppressor genes, depending on whether their mutant phenotypes are dominant or recessive, respectively.
There are a number of viruses known to be involved in human as well as animal cancer. Of particular interest here are viruses that do not contain oncogenes themselves; these are slow-transforming retroviruses. Such viruses induce tumours by integrating into the host genome and affecting neighboring protooncogenes in a variety of ways. Provirus insertion mutation is a normal consequence of the retroviral life cycle. In infected cells, a DNA copy of the retrovirus genome (called a provirus) is integrated into the host genome. A newly integrated provirus can affect gene expression in cis at or near the integration site by one of two mechanisms. Type I insertion mutations up-regulate transcription of proximal genes as a consequence of regulatory sequences (enhancers and/or promoters) within the proviral long terminal repeats (LTRs). Type II insertion mutations located within the intron or exon of a gene can up-regulate transcription of said gene as a consequence of regulatory sequences (enhancers and/or promoters) within the proviral long terminal repeats (LTRs). Additionally, type II insertion mutations can cause truncation of coding regions due to either integration directly within an open reading frame or integration within an intron flanked on both sides by coding sequences, which could lead to a truncated or an unstable transcript/protein product. The analysis of sequences at or near the insertion sites has led to the identification of a number of new protooncogenes.
With respect to lymphoma and leukemia, retroviruses such as AKV murine leukemia virus (MLV) or SL3-3 MLV, are potent inducers of tumours when inoculated into susceptible newborn mice, or when carried in the germline. A number of sequences have been identified as relevant in the induction of lymphoma and leukemia by analyzing the insertion sites; see Sorensen et al., J. Virology 74:2161 (2000); Hansen et al., Genome Res. 10(2):237-43 (2000); Sorensen et al., J. Virology 70:4063 (1996); Sorensen et al., J. Virology 67:7118 (1993); Joosten et al., Virology 268:308 (2000); and Li et al., Nature Genetics 23:348 (1999); all of which are expressly incorporated by reference herein. With respect to cancers, especially breast cancer, prostate cancer and cancers with epithelial origin, the mammalian retrovirus, mouse mammary tumour virus (MMTV) is a potent inducer of tumours when inoculated into susceptible newborn mice, or when carried in the germ line. Mammary Tumours in the Mouse, edited by J. Hilgers and M. Sluyser; Elsevier/North-Holland Biomedical Press; New York, N.Y.
The pattern of gene expression in a particular living cell is characteristic of its current state. Nearly all differences in the state or type of a cell are reflected in the differences in RNA levels of one or more genes. Comparing expression patterns of uncharacterized genes may provide clues to their function. High throughput analysis of expression of hundreds or thousands of genes can help in (a) identification of complex genetic diseases, (b) analysis of differential gene expression over time, between tissues and disease states, and (c) drug discovery and toxicology studies. Increase or decrease in the levels of expression of certain genes correlate with cancer biology. For example, oncogenes are positive regulators of tumorigenesis, while tumour suppressor genes are negative regulators of tumorigenesis. (Marshall, Cell, 64: 313-326 (1991); Weinberg, Science, 254: 1138-1146 (1991)).
Immunotherapy, or the use of antibodies for therapeutic purposes has been used in recent years to treat cancer. Passive immunotherapy involves the use of monoclonal antibodies in cancer treatments. See for example, Cancer: Principles and Practice of Oncology, 6th Edition (2001) Chapt. 20 pp. 495-508. Inherent therapeutic biological activity of these antibodies include direct inhibition of tumour cell growth or survival, and the ability to recruit the natural cell killing activity of the body's immune system. These agents are administered alone or in conjunction with radiation or chemotherapeutic agents. Rituxan® and Herceptin®, approved for treatment of lymphoma and breast cancer, respectively, are two examples of such therapeutics. Alternatively, antibodies are used to make antibody conjugates where the antibody is linked to a toxic agent and directs that agent to the tumour by specifically binding to the tumour. Mylotarg® is an example of an approved antibody conjugate used for the treatment of leukemia. However, these antibodies target the tumour itself rather than the cause.
An additional approach for anti-cancer therapy is to target the protooncogenes that can cause cancer. Genes identified as causing cancer can be monitored to detect the onset of cancer and can then be targeted to treat cancer.
SEMA4D is a 150 kDa surface antigen that belongs to the semaphorin family (Herold C. et al., (1995) Int Immunol; 7(1):1-8). The membrane bound form of SEMA4D has an N-terminal signal sequence followed by a Sema domain, an Ig-like domain, a lysine rich region, a hydrophobic trans-membrane domain and a cytoplasmic tail (Hall K T et al., (1998) PNAS, 93(21):11780-5). The extracellular domain conatind putative N-linked glycosylation sites. The cytoplasmic domain contains a site for tyrosine phosphorylation and multiple sites for serine phosphorylation. SEMA4D forms homodimers, C647 in the Sema domain is required for dimerisation. Shedding of the extracellular domain is metalloprotease dependent, and is regulated by phosphorylation (Elhabazi A. (2001) J Immunol; 166(7):4341-70). Receptors for SEMA4D include Plexin-B1 (Vikis H G (2000) PNAS 97(23):12457-62 and CD72 (Kumanogoh A (2000) Immunity, 13(5):621-31).
Members of the semaphorin family are involved in axonal guidance, and SEMA4D is thought to have a role in the regulation of the humoral and cellular immune response. SEMA4D knock-out mice show defects in the immune system. It has been suggested that, given its role in of the immune response, SEMA4D could be used to stimulate T-cells and B-cells in cancer patients (JP2001048803 and WO 97/17368). SEMA4D is also involved in angiogenesis (Basile J R (2004) Cancer Res; 64(15):5212-24), and may have a role in invasive cell growth (Conrotto P et al., (2004) Oncogene; 23(30):5131-7 and Giordano S. et al., (2002) Nat Cell Biol; 4(9):720-4). SEMA4D has also been implicated in T-cell, but not B-cell, non-Hodgkin's lymphoma (Dorfman D M et al., (1998) Am J Pathol; 153(1):255-62).